Remove lowest_layer, add get_lowest_layer (API Change):

fix #1417

* New get_lowest_layer free function works for any object
* New lowest_layer_type trait works for any object
* New examples and documentation on layered streams

API Changes:

* The member function lowest_layer is removed from all
  types provided by the library:

Actions Required:

* Call the free function get_lowest_layer instead of
  member lowest_layer.

example/websocket/server/sync-ssl/CMakeLists.txt

@@ -22,7 +22,7 @@ if (OPENSSL_FOUND)
     set_property(TARGET websocket-server-sync-ssl PROPERTY FOLDER "example-websocket-server")
 
     target_link_libraries (websocket-server-sync-ssl
-        ${OPENSSL_LIBRARIES}
+        OpenSSL::SSL OpenSSL::Crypto
         )
 
 endif()